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H. Zou, L. Chu, Yan Wang
This study demonstrates a bioelectrochemical system (BES) combining biofilm cultivation on activated carbon fiber (ACF) with electrolysis for methyl orange (MO) azo dye wastewater treatment. Operating at optimized parameters (2.0 V applied voltage, 20 mg/L MO, 0.5 g/L glucose, pH 8.0, 3 d HRT), the system achieved 81.9% color removal efficiency. The wide effective pH range (6–9) and complete cleavage of azo bonds (–N=N–) demonstrate practical applicability for industrial dye wastewater treatment with reduced energy consumption compared to electrochemical methods alone.
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Azo dye wastewater treatment is urgent necessary nowadays. Electrochemical technologies commonly enable more effi cient degradation of recalcitrant organic contaminants than biological methods, but those rely greatly on the energy consumption. A novel process of biofi lm coupled with electrolysis, i.e., bioelectrochemical system (BES), for methyl orange (MO) dye wastewater treatment was proposed and optimization of main infl uence factors was performed in this study. The results showed that BES had a positive effect on enhancement of color removal of MO wastewater and 81.9% of color removal effi ciency was achieved at the optimum process parameters: applied voltage of 2.0 V, initial MO concentration of 20 mg/L, glucose loads of 0.5 g/L and pH of 8.0 when the hydraulic retention time (HRT) was maintained at 3 d, displaying an excellent color removal performance. Importantly, a wide range of effective pH, ranging from 6 to 9, was found, thus greatly favoring the practical application of BES described here. The absence of a peak at 463 nm showed that the azo bond of MO was almost completely cleaved after degradation in BES. From these results, the proposed method of biodegradation combined with electrochemical technique can be an effective technology for dye wastewater treatment and may hopefully be also applied for treatment of other recalcitrant compounds in water and wastewater. Azo dye wastewater treatment in a novel process of biofi lm coupled with electrolysis 39 wastewater by using BES in this study. In the BES designed here, a stainless steel column was used as the cathode, where an activated carbon fi ber (ACF) was attached to the surface of cathode to enrich microorganisms. The performance of BES was assessed in terms of color removal effi ciency of MO. The effect of applied voltage, MO concentration, carbon source content and pH on MO denitrifi cation rate was investigated to optimize the operation on BES. Moreover, to investigate the change of molecular and structural characteristics during the MO treatment, UV/Visible absorption spectra with reaction time was further analyzed. These results obtained from this study, BES linking biological with electrochemical process, may serve as a new suggestion for the treatment of dye wastewaters or non-biodegradable industrial wastewaters. Material and methods Experimental setup The BES was made from polyvinyl chloride with a single-chamber cylinder and Figure 1 shows the schematic diagram of BES adopted in this study. The BES had a total and working volume of 4.0 L and 3.0 L respectively with an internal diameter of 16 cm and 20 cm in height. The BES consisted of an ACF (Shanghai Zhaokuo, Co., Ltd, China) wrapped around the stainless steel column (6 cm internal diameter × 12 cm height × 0.15 cm wall thickness) as the cathode electrode and a high-purity graphite rod (2 cm diameter × 13 cm length) as the anode electrode. An adjustable direct current regulated power supply (PS-305DM; Dongguan Longwei Electronic Technology, Co., Ltd, China) was connected with anode and cathode to provide voltage. Besides, an automatic stirrer (OS20; Beijing Dragon Laboratory Instruments Limited, China) was installed at the top of the BES to provide well-mixed environment. Experimental design After construction, the experiments were carried out for 167 days. Activated sludge (1 L, 1 g/L of MLSS), i.e., seed sludge, was collected from an oxidation ditch confi guration (Fengyang Municipal Wastewater Treatment Plant, Anhui, China) and immediately washed three times using deionized water to remove soluble carbon sources. And then, it was inoculated in the BES reactor to accelerate the biofi lm development onto the surface of ACF cathode, including three stages: fi rstly, from day 0 to day 20, a single synthetic wastewater was fed to the BES to promote the rapid growth of microorganisms; secondly, after that, a 1:1 (vol/vol) mixture of synthetic wastewater and azo dyes wastewater containing 30 mg/L MO was fed for 15 days to gradually enrich the specifi c microorganisms; fi nally, from day 36, it was intensively enriched by feeding the only MO wastewater for 30 days. In order to investigate the effect of process parameters on MO color removal in BES, the applied voltage, MO concentration, carbon source content and pH were gradually increased respectively (see Table 1) after biofi lm formation. During the experimental period, the hydraulic retention time (HRT) was maintained at 3 d according to the preliminary test. The synthetic wastewater consisted of organic carbon, nutrients and buffer solution and its composition is as follows: 40 mg/L KH2PO4, 40 mg/L (NH4)2SO4, 3 mg/L CaCl2, 45 mg/L MgSO4∙7H2O and 1 mL/L of nutrient solution (1200 mg/L FeCl3∙6H2O, 130 mg/L H3BO3, 25 mg/L CuSO4∙5H2O, 160 mg/L KI, 100 mg/L MnCl2∙4H2O, 50 mg/L Na2MoO4∙2H2O, 110 mg/L ZnSO4∙7H2O, 130 mg/L CoCl2∙6H2O and 800 mg/L EDTA). In addition, glucose and MO were added into the synthetic wastewater according to the experimental arrangement, which was used as the carbon source. The other compositions acted as nutrient and buffer solution for microbial growth. Analytical methods Samples of effl uents were fi ltered through a 0.22 μm-pore-size syringe fi lter prior to analysis. MLSS analysis was performed according to the standard methods (APHA, 2005). pH was measured by a pH meter analyzer (S20, Mettler-Toledo, Switzerland). Absorbance was analyzed by measuring the adsorption at 463 nm using an UV-3600 (Shimadzu, Japan). Fig. 1. Schematic diagram of BES 40 H. Zou, L. Chu, Y. Wang Results and discussion Effect of applied voltage on color removal The performance of color removal in BES at different applied voltages (HRT: 3 d; applied voltage: 0, 0.6, 0.8, 1.0, 1.4, 1.8, 2.2, 2.5 and 3.0) are shown in Figure 2. MO concentration of infl uent was maintained at 20 mg/L. It is clearly observed from Figure 2 that color removal effi ciency increased with the increasing voltage applied from 0 V to 2.5 V, displaying the promoting effect of applied voltage on color removal. The BES is totally related to the current density and it was enhanced with the increasing voltage applied, providing suitable conditions for microorganism on the biofi lm and electrochemical reaction (Chen et al. 2015). The highest color removal effi ciency (80.5%) was observed in BES at an applied voltage of 2.5 V. The reason might be that the presence of current density was more conductive to the bacteria growth and electrochemical reaction. At 3.0 V, it was declined to 77.2%. This can be due to the fact that high concentrations of intermediate products would be formed in solution at an excessive applied voltage. There exists an inevitable comparison for electron between the further degradation of intermediate products and the rupture of –N=N– at the cathode’s surface (Liu et al. 2015), leading to the decline in current effi ciency. Notably, the color removal effi ciency was 68.9% at 1.8 V signifi cantly higher than that at 1.4 V (53.5%) in BES. This may be attributed to the oxidative electrolysis of water and reduction of protons (Thrash and Coates 2008), thus producing more oxygen at anode and hydrogen at cathode, which were utilized by microorganisms as electron acceptor and electron. Table 1. Experimental design used here Process parameters Set value Applied voltage 0, 0.6, 0.8, 1.0, 1.4, 1.8, 2.2, 2.5 and 3.0 V MO concentration 5, 10, 20, 40, 60, 80, 100 mg/L Carbon source content 0, 0.1, 0.3, 0.5, 0.7, and 1.0 g/L pH 3, 4, 5, 6, 7, 8, 9, 10, and 11 Fig. 2. Color removal effi ciency at different applied voltages in BES Fig. 3. Color removal effi ciency with different dye concentration of infl uent in BES As the BES was controlled under no electric fi led, i.e., applied voltage of 0 V, exhibiting only a typical biological reaction, the color removal effi ciency was rather low (21.2%). Dye wastewater is well known to be of low biodegradability, which was hard to be treated by using only a single biological treatment method. And then, it was sharply increased up to 36.8% at 0.6 V, suggesting that micro-current fl owing through the biofi lm on the cathodic surface had positive effect on the microbial metabolism due to the electric fi eld stimulation. Effect of dye concentration on color removal The effects of different MO concentration of infl uent (5, 10, 20, 40, 60, 80, 100 mg/L) on color removal performance are shown in Figure 3. During the treatment process, BES was operated at the same condition (HRT: 3 d; applied voltage: 2.0 V). Color removal effi ciency gradually decreased from 90.2% to 42.8% with the increase of initial MO concentration ranging from 5 mg/L to 100 mg/L, displaying a negative effect of initial dye loading on BES, which may be most likely due to the toxicity of the dye metabolites (such as aromatic amines) produced during dye reduction at high dye concentrations (Pearce et al. 2003). Similar results were also found in a report (Sponza and Işik 2005) that increase in Direct Black 38 concentration caused a decrease in color removal effi ciency in an anaerobic/aerobic sequential reactor responsible for dye wastewater treatment. These results suggested that the biomass inhibition effect could occur in BES when the dye concentration exceeded a proper range. Effect of carbon source content on color removal Figure 4 shows the effect of different carbon source content (0, 0.1, 0.3, 0.5, 0.7, and 1.0 g/L), glucose used here, on the color removal performance in BES. The other operation conditions were listed as follows: HRT=3 d, applied voltage=2.0 V, initial MO concentration=20 mg/L. It was observed that glucose displayed an obvious promotion on color removal effi ciency in BES. The color removal effi ciency was only 36.2% without addition of carbon source and it increased signifi cantly up to 59.5% with the addition of 0.1 mg/L of glucose. This result was consistent with other studies (Al-Amrani et al. 2013, Murali et al. 2013), where co-subs